In our country, the service of IMT-2000 (International Mobile Telecommunication 2000) was started in October 2001 before the rest of the world, and thus the transmission and access technique in a mobile communication system is rapidly developing. In addition, the technique of HSDPA (High Speed Down-link Packet Access) and the like are standardized and the data transmission at about 10 Mbps at maximum is now being put to practical use.
On the other hand, the standardization to realize a broad band wireless Internet access that targets a transmission rate of 10 Mbps to 100 Mbps is in progress and various techniques have been proposed. The condition required to realize high transmission rate wireless communication is the improvement of frequency usage efficiency. Since the transmission rate and the used bandwidth are in a proportional relationship, a simple solution to increase the transmission rate is to widen the frequency bandwidth to be used. However, available frequency bands are in a tight situation and it is unlikely that a sufficient bandwidth is allocated when constructing a new wireless communication system. Consequently, it becomes necessary to improve the frequency usage efficiency.
In addition, another required condition is to provide service in a private area (isolated cell), such as a wireless LAN, in a seamless manner while realizing service in a communication area constituted by cells, such as a mobile phone.
Techniques having the possibility of solving these problems include a technique called one-cell reuse OFDM/(TDMA, FDMA) (Orthogonal Frequency Division Multiplexing/Time Division Multiple Access, Frequency Division Multiple Access). This is a technique in which communication is performed using the same frequency in all of the cells in a communication area constituted by cells, the modulation scheme when performing communication is the OFDM, and the access scheme uses the TDMA and FDMA. This is a communication system, without doubt, capable of realizing higher-speed data communication in an isolated cell while maintaining a common wireless interface with a cell area.
The OFDM, TDMA, and FDMA, which are constitutional techniques of the OFDM/(TDMA, FDMA), are explained briefly.
First, the OFDM is a technique used for IEEE802.11a, which is a wireless system of 5 GHz band, and a terrestrial digital broadcasting. The OFDM is a system in which tens to thousands of carriers are arranged at intervals of a minimum frequency that does not cause interference theoretically and communication is performed simultaneously. In the normal OFDM, such a carrier is called a sub-carrier and each sub-carrier is modulated when performing communication by a modulation scheme, such as the PSK, QAM, etc. Further, with an error correction technique combined, it has grate resistance to frequency selective fading. In the present specification, the number of sub-carriers used in the OFDM is assumed to be 768.
Next, the TDMA is an access system in which time is divided when transmitting/receiving data. Normally, in a communication system using the TDMA as an access system, a frame configuration is used in which there are a plurality of slots, which is a unit of communication time, and further, it is general to allocate a control slot necessary for receiving a frame at the front of the frame in the case of Down-link. In the present specification, it is assumed that a frame is composed of nine slots and the front slot is allocated as a control slot.
Next, the FDMA is an access system in which frequencies are divided when transmitting/receiving data. Normally, in a communication system using the FDMA as an access system, frequencies are divided into several bands, which are frequency bands for performing communication, and thus terminals (mobile station apparatus) that access are classified. Normally, a protective band called a guard band is prepared between divided frequency bands, however, in the OFDM/(TDMA, FDMA), no guard band is used because the frequency usage efficiency is decreased, or if used, its band is very narrow, just for accepting several sub-carries. In the present specification, 768 sub-carries used in the OFDM are divided into 12 groups, each group including 64 sub-carries, for performing the FDMA.
Next, the OFDM/(TDMA, FDMA) is explained based on the above-described introduction. FIG. 42 is a diagram showing a two-dimensional frame configuration of the OFDM/(TDMA, FDMA). In FIG. 42, the vertical axis represents the frequency and the horizontal axis represents the time. One of a plurality of rectangles shown in FIG. 42 is the minimum unit used for data transmission, composed of a plurality of OFDM symbols, and is referred to as a slot in the present specification. Among the slots, those with diagonals are control slots. In this case, the figure means that there are nine slots in the time direction and 12 slots in the frequency direction in one frame, that is, there exist a total of 108 slots (among then, 12 slots are control slots) in one frame. In addition, in the present specification, a group of slots in the direction of the frequency axis at the same time (composed of 12 slots in the case of FIG. 42) is referred to as a time channel and a group of slots in the direction of the time axis at the same frequency (composed of nine slots in the case of FIG. 42) is referred to as a frequency channel. In form, a slot is denoted by (Tn, Fm), a time channel is denoted by Tn (n is a natural number from 1 to 9), and a frequency channel is denoted by Fm (m is a natural number from 1 to 12). For example, the hatched slot in FIG. 42 is a slot denoted by (T4, F7).
Next, communication from a base station (referred to as AP or base station apparatus) to a mobile station (referred to as MT, mobile station apparatus, or simply “terminal”) is considered. When an AP allocates data of 15 slots to an MT, it is assumed that the data is allocated to the slots with vertical lines in FIG. 42, although there may be various cases. In other words, the data to be received by the MT is allocated to (T2 to T4, F1), (T5 to T8, F4), and (T2 to T9, F11). Further, in order to indicate that the AP has allocated data to the MT, it is necessary to embed data indicative of the allocation to the control slot of the frequency to be used. In the case of this example, (T1, F1), (T1, F4), and (T1, F11) correspond to the control slots.
The OFDM/(TDMA, FDMA) system is a system in which a plurality of mobile stations transmit and receive data to and from the base station by changing the frequency and time based on those described above. In FIG. 42, the figure is drawn such that there seems to be a gap between slots for convenience's sake, however, the presence or absence of a gap is of no importance.
FIG. 43 is a block diagram showing a general configuration of a transmission circuit used in the OFDM/(TDMA, FDMA). The transmission circuit shown in FIG. 43 has a data multiplex part 431. In addition, the transmission circuit has 12 error correction encoding parts 432-a to 432-1 and at the same time, has 12 serial/parallel conversion parts (S/P conversion parts) 433-a to 433-1. A transmission power control part 435 exhibits a function of changing transmission power for each frequency channel.
In the data multiplex part 431, information data is separated into 12 groups in units of packets for transmission. In other words, the data multiplex part 431 physically specifies the ODFM/(TDMA, FDMA) slot to be specified by a module, such as a CPU etc., not shown schematically here. After that, error correction encoding is performed in the error correction encoding parts 432-a to 432-1, separation into 64 groups is performed in the S/P conversion parts 433-a to 433-1, and each carrier is modulated in a mapping part 434. In the transmission power control part 435, power control is performed into transmission power for each sub-channel specified by a module, such as a CPU etc., not shown schematically and IFFT (Inverse Fast Fourier Transform) processing is performed in an IFFT part 436. When generating an OFDM signal of 768 waves, the number of points of the IFFT normally used is 1,024.
After that, in a P/S conversion part 437, conversion into serial data is performed and then a guard interval is inserted in a guard interval insertion part 438. A guard interval is inserted in order to reduce interference between symbols when receiving an OFDM signal. Then, after converted into an analog signal in a D/A conversion part 439 and converted into a frequency for transmission in a radio transmission part 440, the data is transmitted from an antenna part 441.
In addition, FIG. 44 is a block diagram showing a general configuration of a reception circuit used in the OFDM/(TDMA, FDMA). The reception circuit shown in FIG. 44 has a data demultiplex part 461 and further, has 12 error correction decoding parts 460-a to 460-1. In addition, the reception circuit has 12 parallel/serial conversion parts (P/S conversion parts) 459-a to 459-1.
In the reception circuit, an operation reverse to that of the transmission circuit is performed basically. The frequency of the radio wave received by an antenna part 451 is converted into a frequency in a frequency band in which A/D conversion is possible by a radio reception part 452. With data having been converted into a digital signal in an A/D conversion part 453, the OFDM symbols are synchronized in a synchronization part 454 and the guard interval is removed in a guard interval removal part 455. After that, the data is paralleled into 1,024 data in an S/P conversion part 456.
After that, the FFT of 1,024 points is performed in an FFT (Fast Fourier Transform) part 457 and demodulation of the sub-carrier of 768 waves is performed in a propagation channel estimation and demapping part 458. Normally, propagation path is estimated by the receiver by sending a known signal from the transmitter to the receiver. After that, the necessary data is serialized in the P/S conversion parts 459-a to 459-1, error correction is performed in the error correction decoding parts 460-a to 460-1, and the data is input to the data demultiplex part 461. In the data demultiplex part 461, the data is processed into information data and output.
Next, the outline of a communication system consisting of cells is explained. FIG. 45(a) is an example of the case where cells have a hexagonal shape and seven frequency bands are used. A base station is arranged in the center of the cell and in cell B0, communication is performed using a frequency band Fc0, in B1, Fc1 is used, and similar combinations follow in the rest of the cells. In such a cell configuration in which the number of frequency bands is sufficient, it is unlikely that the adjacent cells use the same frequency and it is possible to perform communication in an excellent condition without influence from the adjacent cells.
FIG. 45(b) is an example of the case where one-cell reuse OFDM/(TDMA, FDMA) is used. Similarly, the configuration consists of hexagonal cells, however, the same frequency Fc0 is used. Consequently, when the one-cell reuse OFDM/(TDMA, FDMA) operates ideally, it follows that a frequency usage efficiency seven times that compared to the case in FIG. 45(a) can be attained. As a result, it can be said that realization of one-cell reuse is a indispensable technique in order to realize high speed communication.
As obvious also from FIG. 45(b), the point that affects the ideal operation of the one-cell reuse is to prevent interference from other cells. Two techniques can roughly be thought as a method for preventing interference from other cells. One method is to establish a communication system in which each terminal removes radio waves from other cells (interference removal) and the other method is to prevent interference from affecting as much as possible. Among these methods, the following two are explained with respect to a specific technique of the latter method.
First, a wireless data communication system, a wireless data communication method, and its program disclosed in Japanese Unexamined Patent Publication No. 2003-18091 (Patent document 1) are explained. A cell configuration relating to the invention described in Patent document 1 is shown in FIG. 46. In FIG. 46, a hexagon constructed by the dotted-line is shown in each cell in comparison with FIG. 45(a). This means that one cell is divided into two areas, one being near the base station and the other, distant therefrom. When cell B0 is focused on, communication is performed conventionally using a frequency Fc0 with terminals in the area distant from the base station and communication is performed using Fc1 to Fc6 with terminals in the area near the base station. It is described that this increases the frequency usage efficiency. Further, it is explained that the use of a sector antenna within an area surrounded by the dotted-line increases the efficiency. This is a technique that utilizes the fact that even the use of the frequency bands Fc1 to Fc6 does not affect the adjacent cells because it is possible to lower transmission power when performing communication with terminals near the base station.
Next, a mobile communication system, a base station apparatus, and a control method of a mobile communication system disclosed in Japanese Unexamined Patent Publication No. 2003-46437 (Patent document 2) are explained. A cell configuration relating to the invention disclosed in Patent document 2 is shown in FIG. 47. In FIG. 47, two hexagons constructed by the dotted-line are shown in each cell in comparison with FIG. 45(a). When cell B0 is focused on, the area most distant from the base station is denoted by Ts1, the second most distant area is denoted by Ts2, and the nearest area is denoted by Ts3. Ts represents time and Ts1 to Ts3 constitute one frame. This means that in B0, transmission power is increased to the maximum during Ts1 and communication is performed, and then, the transmission power is lowered in Ts2 and Ts3 and communication is performed. Similarly, each cell performs communication by changing transmission power in accordance with the time, respectively.
In B0, when communication is being performed with increased transmission power during Ts1, communication with transmission power increased to the maximum is not being performed in other adjacent cells, and therefore, it is possible to perform communication in B0 in a state in which interference from other cells is small. For the cells B1 to B6, the same advantage is secured similarly.
Patent document 1: Japanese Unexamined Patent Publication No. 2003-18091
Patent document 2: Japanese Unexamined Patent Publication No. 2003-46437